A prior art scanning-beam microwave landing system (hereinafter abbreviated to MLS") is described in detail in an article by H. W. Redlien and R. J. Kelly, "Microwave Landing System: the New International Standard", Advances in Electronics and Electron Physics, Vol. 57, Academic Press, Inc., 1981 (ISBN 0-12-014657-6), pages 311-410. Accordingly, said article is hereby incorporated by reference in its entirety, as are the other references cited therein. The following passages should be particularly noted:
Pages 311-410 (description of the principle), PA1 Page 330 (description of the sequence of signals radiated from the individual stations), PA1 Pages 340-349 (description of the MLS ground station), PA1 Pages 349-357 (description of the MLS airborne equipment), and PA1 Page 404 (description of the monitoring function).
In radio navigation systems such as the MLS described in the above-cited Redlien et al article, it is very important that the radiated signals remain within certain specified values. To monitor this, the MLS contains field monitors, antenna aperture (integral) monitors, and internal monitors.
The field monitors monitor whether the radiated signals meet certain specifications. A distinction is made between near-field monitors (which are located in the immediate vicinity of the station to be monitored) and far-field monitors (which are typically located at a distance of at least several hundred meters from the station to be monitored). The ground equipment of a basic (prior art) MLS consists of an elevation station and an azimuth station. At larger airports, there may also be provided a back azimuth station, which provides azimuth guidance after a missed approach.
The MLS operates at very high frequencies (typically in the 5 GHz range) which make it necessary to take special precautions to eliminate unwanted reflections from other objects within the systems's effective coverage area. Such an "object" may even be the antenna of a far-field monitor, for example.